The problem of how information is stored in the brain is fundamental for neurobiology. The overall goal of our research is to elucidate whether the mechanism of learning and memory involves structural modifications of synaptic connections. Although historically it has been believed to be the case, there is no unequivocal experimental evidence to support such a notion. The results of previous electron microscopic studies of learning-induced synaptic restructuring in the vertebrate brain need to be reevaluated since they were obtained using biased technical approaches. We now propose to reexamine the issue with the aid of modern unbiased techniques for synapse quantitation. Recently, we identified a novel morphological subtype of synapses characterized by multiple, completely partitioned transmission zones. The efficacy of impulse transmission is expected to be unusually high in a synapse having several transmission zones, rather than only one, as is usual. Since an augmented synaptic efficacy is widely regarded as a basic mechanism of learning and memory, we developed a model of learning-induced structural synaptic plasticity, the pivotal element of which is the formation of additional synapses with multiple transmission zones from other preexisting synaptic junctions. The proposed experiments are designed to test the predictions of this model by examining alterations in synaptic efficacy and ultrastructure associated with classical conditioning. We will utilize the paradigm of trace conditioning of the rabbit nictitating membrane response which is crucially dependent on the integrity of the hippocampal formation. Eyeblink conditioning results in an augmented synaptic responsiveness of the majority of hippocampal neurons, and this behavioral paradigm offers the important opportunity to differentiate learning-specific from activity-related changes by comparing conditioned, pseudoconditioned and naive animals. Hippocampal slices maintained in vitro will be used to detect conditioning-specific alterations in synaptic potentials with extracellular recordings and in the number of synaptic contacts with the unbiased stereological disector technique. The results to be obtained will unequivocally show if classical conditioning is accompanied by changes in the number of any morphological subtype of synapses and if these changes correlate with conditioning-specific alterations in synaptic efficacy. Such data are essential for a better understanding of mechanisms of learning and memory, as well as of the pathogenesis of memory disorders such as Alzheimer's disease.